"Oh shit," the man says a few times, as two police officers get out of their patrol car and approach another man, who is shouting and holding a knife, on the street. "They got their guns out."

Then, a series of rapid gunshots. Nine bullets.

"Oh my god," says the man, who had filmed what happened on his cell phone. "They just killed him."

The witness captured the shooting death of 25-year-old Kajieme Powell by two St. Louis police officers outside a convenience store in August, 10 days after a white police officer shot and killed Michael Brown in a nearby suburb. Police released the footage a day later in the name of transparency. But the disturbing video, which you can see here, prompted more questions instead. People who watched the footage were shocked at how quickly the officers shot Powell, barely 20 seconds after they arrived on scene.

St. Louis Police officials defended the shooting. "If someone approaches you within three feet with a knife, I think you have a right to protect yourself," Police Chief Sam Dotson said back then. "I think we can all understand what is going on in Ferguson, but I think every police officer that's out here has the right to defend themselves and the community."

And every U.S. police officer does have that right, according to the legal standard created by a pair U.S. Supreme Court rulings in the 1980s. Police officers cannot justify use of deadly force against someone based on whether the officers' intentions were good. They must show that their actions were "objectively reasonable." Would another officer with similar training and experience, if placed in the same circumstances, have perceived that person as a threat too, and acted the same way?

That's the law, and that's the policy of nearly every police department in America. But there's something that no court ruling or law enforcement handbook can truly standardize: a police officer's discretion.

The word "discretion" is often used to describe the seconds right before an officer reaches for his weapon, whether it's a baton, a Taser, or a gun. It's the adrenaline-filled moment when an officer believes there is a threat—regardless of whether it actually exists—that poses a danger to himself or others around him. He knows his training, he knows his department policy. He can only use physical force if the suspect doesn't respond to verbal warnings, and he can only use lethal force as a last resort.

A police officer's judgment, however, isn't the same in every situation. And this year's high-profile officer-involved deaths have some people wondering whether they can trust it. For some officers, discretion can override training and policy.

"Once I say to you, 'You're trained, you've got a policy, but use your discretion,' I have let you loose on the world—as hopefully a really smart, decent cop who wants to do the right thing," says John Firman, director of development at the International Association of Chiefs of Police.

It's important to note that Michael Brown and Kajieme Powell and Tamir Rice and John Crawford are exceptional cases. More than 900,000 police officers interact with citizens every day without using excessive or lethal force. Like many law enforcement policies, the Ferguson Police Department's use of lethal-force policy, shared with National Journal in August, states that "the department recognizes and respects the value and special integrity of each human life."

"We do not shoot because we're judge and jury and we decided a death sentence is adequate," says Richard Weinblatt, the dean of public services at Ivy Tech Community College in Indianapolis and a former police chief. "If an officer is doing it on that basis, then trust me, 99% of the officers out there do not want that particular officer in law enforcement because they make all the rest of us look bad, and they are not keeping with their oath of office."

But these deaths expose an aspect of policing that is extremely difficult to predict and study, one that leaves a living officer to tell his story and a dead victim who can't. They also reveal a painful divide between police and populace, and a forgiving society that tends to side with the officer over the suspect.

"Not every police officer is a paragon of mental health and stability," says Laurence Miller, a clinical and forensic psychologist in Florida who serves as the police psychologist for the West Palm Beach Police Department.

Officer discretion has received closer scrutiny in the last two decades, in large part because of the filmed beating of Rodney King by white Los Angeles police officers in 1991, says Terry Gilbert, a civil-rights lawyer in Cleveland and a board member of the National Police Accountability Project. "People now understood that police are capable of making bad judgments, or worse, actually engaging in criminal acts," he says.

Cops are, after all, human. Their discretion can save lives, but it can also be dead-wrong. A toy gun can be mistaken for the real thing. A man reaching for his waist could be looking for the ID in his wallet, not a weapon. Sometimes, officers can indirectly escalate a situation—not waiting for backup or finding cover—that ultimately pushes them to use force, says civil-rights attorney John Burris.

"Officers create dangerous situations for themselves, and then they try to shoot their way out of it," says Burris, who served as co-counsel in the King case. He also led a wrongful-death suit in the fatal shooting of Oscar Grant by BART police officers in Oakland, Calif., in 2009. But "the courts don't care if the police created the danger and heightened the possibility for a deadly encounter by their actions."

Some officers—the ones Weinblatt would say make the rest look bad—allow their emotions to dictate their discretion. "Sometimes it's the police officer's own personality and temperament, Miller says. "Sometimes it's just an incorrigible suspect who goads or provokes the police officer."

One veteran officer gave this advice for citizens who might challenge law enforcement, in an August op-ed in The Washington Post: "If you don't want to get shot, tased, pepper-sprayed, struck with a baton, or thrown to the ground, just do what I tell you. Don't argue with me, don't call me names, don't tell me that I can't stop you."

Officer discretion can also be influenced by something much less traceable: implicit bias. Miller explains that if an officer perceives, for example, black men as more likely to be dangerous, he will be more vigilant, and his threshold to act will be lower. He will be more likely to perceive a black suspect as a threat, thus creating the legal basis necessary to use force.

"When an officer like Darren Wilson said he felt threatened by 'Hulk Hogan,' the question is, did he feel threatened because he was dealing with a large black man?" says Marc Morial, president of the National Urban League and a former mayor of New Orleans. " 'Reasonableness' does not include fear because your mind is filtered with stereotypes."

Still, Americans, like the ones who sat on the grand juries in Staten Island, N.Y., and Ferguson, Mo., tend to say that police use of deadly force is "objectively reasonable." "But when society weighs whether the deaths of black men at the hands of police are reasonable, it does so with the additional burden of American beliefs about black criminality, black super-strength, black dangerousness," Adam Serwer wrote in BuzzFeed this week after the white police officer who killed Eric Garner in a choke hold was not indicted on criminal charges. "On the other side are our collective beliefs about police, seen as more noble, more selfless, and more resistant to all-too-human flaws like wrath or deceit."

The average juror, Burris says, doesn't want to believe that a cop would kill someone for no reason. "People want to believe that the police do good work. For the most, part they do." he says. "They don't want to believe that there's discriminatory aspects in police. If you're white, you don't have to."

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